Make sdl2 compatible with base < 4.11

[ivanperez-keera]

The module SDL.Input.Mouse assumes that (<>) is defined in Prelude, but that only happened in base 4.11. Until then, the symbol was defined in Data.Semigroup, and another is defined in Data.Monoid (for some types, the two are equivalent).

It's being used here: https://github.com/haskell-game/sdl2/blob/master/src/SDL/Input/Mouse.hs#L307

We are trying to move dunai to use sdl2, and we are hitting a problem with supporting older versions of GHC.

Would it be possible to add a conditional import so that sdl2 compiles with older versions of GHC as well?

Since <> is used on Strings, which are lists of characters, and lists are monoids, then it's safe to pick the version from Data.Monoid because they are equivalent per the definition of Semigroup:

class Semigroup a where
  (<>) :: a -> a -> a

  default (<>) :: Monoid a => a -> a -> a
  (<>) = mappend

Therefore, something along the following lines would do:

#if !MIN_VERSION_base(4,11,0)
import Data.Monoid ((<>))
#endif

[ocharles]

base-4.11 is over 4 years old. Pre 4.11 puts you all the way back to GHC 8.2. I'm not sure we've ever decided what our support policy is, but that is a whopping huge window and not sure we have the resources to actually commit to that

[ivanperez-keera]

From the current version of the same file in sdl2:

sdl2/src/SDL/Input/Mouse.hs

Lines 67 to 69 in 5c92d46

	#if !MIN_VERSION_base(4,8,0)
	import Control.Applicative
	#endif

Those three lines help support the GHC 7 series (the last version of which was released 9 years ago; incidentally, both dunai and Yampa work well with all versions of GHC since 7.6 and they are tested with those regularly).

I'm not trying to stir a debate on where the line should be drawn. All I'm asking is if you'd accept a 3 line patch in the same file :)

[ocharles]

Of course we could accept such a patch, I'm just saying don't expect it to not just break again :)

[ivanperez-keera]

Thanks!

I've sent a PR. I tried it with GHC 7.10 (since base 4.8, which is currently mentioned, came with GHC 7.10) and it compiles.